Wellbore holes for production of oil and gas from subterranean formations are typically completed by placement of a casing, cementing of the casing, and then perforating the casing to provide communication between the formation and the inside of the casing. When the formation is a poorly consolidated formation, the perforating is followed by insertion of a wire-wrapped screen and a gravel packing between the screen and the inside of the casing. A production tubing is then inserted along with packers, pumps or other artificial lift devices as required to produce fluids from the wellbore.
Temporarily controlling permeability at the borehole surfaces is critical during this completion process. While drilling is proceeding, and throughout the completion process, the hydrostatic pressure of the fluids within the borehole is maintained above the formation pressures. This prevents formation fluids from entering the borehole, displacing the drilling fluid with less dense liquids and resulting in a "kick" or a blow-out. The loss of drilling fluids into the formation is controlled by fluid loss control additives. These additives cause a cake to be formed at the borehole wall which provides a low permeability barrier. This cake keeps the drilling fluids within the borehole in spite of the fluids within the borehole being at pressures greater than the formation pressures. During drilling this barrier generally does not have to be removable. But when a potential oil or gas producing formation is being drilled, it is desirable to have a barrier which is removable.
After the borehole is drilled through the target formations, casings are set and cemented into place. The casing is then perforated, usually by explosive charges which send projectiles from a gun within the casing through the casing and into the formation. These perforations serve as conduits for formation fluids to flow into the borehole when production commences. Until the well completion is finished, fluid loss into these perforations must be minimized in order to prevent damage to formation permeability. This fluid loss must be controlled by a means which does not inhibit formation fluids from entering the casing via the perforations when production commences.
To provide a fluid loss control which can be removed when the need for it is eliminated, liquid biopolymers, natural polymers and their derivatives are typically utilized. Biopolymers include polysaccharides, and xanthan gum. Natural polymers and their derivatives include starch, carboxymethylcellulose (CMC), low molecular weight hydroxyethyl cellulose (HEC), and carboxymethylhydroxyethyl cellulose (CMHEC). These polymers are typically removed by flushing the borehole with acidic compounds. Acids degrade the polymers and permit removal of the barrier. These polymers may be used in conjunction with the salt granules. Salt granules, by themselves, generally do not form a sufficient barrier, but in conjunction with a biopolymer such as starch, are very effective.
The post-flush with acid can degrade polymers which come into contact with the post-flush, but it is difficult to contact a significant portion of the polymer barrier with the post-flush. Once a portion of the barrier is degraded, the fluids will flow into the formation from the break in the barrier. Significant portions of the barriers are bypassed unless the post-flush is continued for an extended period. Extending this post flush for a long period is undesirable due to the cost, delay, and the acids which are then produced from the wellbore as production begins.
It would be preferable to have a polymer breaker which could be generated from an inert precursor, the precursor being present throughout the barrier, and generating the polymer breaker in significant quantities when the barrier is no longer needed. Many compounds which hydrolyze to form acids, such as esters, do so at rates which significantly decrease at even slightly acidic pHs. It would be further advantageous to have a precursor which degraded at predictable rates which did not vary significantly with pH. It would also be advantageous to have such a precursor which is water soluble. A soluble breaker would not be susceptible to physical separation from the effective portion of the fluid loss barrier. Further, a soluble breaker could be present in both the initial fluid loss composition, and subsequent wellbore fluids to ensure that the fluid loss barrier is totally exposed to the breaker.
It is therefore an object of the present invention to provide a well completion method and a fluid loss control composition wherein an effective fluid loss barrier is established from an aqueous mixture of polymers which contains soluble precursors to polymer breakers. It is a further object to provide such a method and composition wherein the release of polymer breakers from the precursors is not strongly pH dependent. It is a further object to provide such a method and composition wherein the fluid loss barrier is effectively removed, resulting in a formation permeability in the vicinity of the perforations which is not reduced by remnants of the fluid loss composition.